1. Field of the Invention
This invention relates to a rotating electric machine comprising an armature having a plurality of salient poles arranged opposite the periphery of a magnet rigidly fitted to a rotary shaft.
2. Background Prior Art
Of various rotating electric machines to be used for electric motors and generators, there is a type of machine comprising an armature arranged opposite the outer periphery of a magnet. Such a rotating electric machine normally has a configuration as illustrated in FIG. 5 of the accompanying drawings.
FIG. 5 illustrates an AC servomotor comprising an inner rotor type DC brushless motor having a concentric winding arrangement. As illustrated, a plurality of semicylindrical magnet segments 2 are rigidly fitted to the outer periphery of a yoke 1 which is integral with a rotary shaft and surrounded by an armature 3 which performs the role of a stator. Armature 3 comprises a plurality of salient poles 4 projecting radially and inwardly toward the segmented magnet 2.
The inner surface 4a of each of the salient poles 4 that faces the segmented magnet 2 a circular arc arranged on an imaginary circle concentrically drawn around the rotary shaft and having a radius of .phi.. The outer surface of each of the magnet segments 2 facing the inner surfaces 4a of the salient poles 4 also exhibits a circular arc having a radius of curvature Rb in an attempt to induce a voltage having a sinusoidal waveform in the winding with a least level of cogging.
With a rotating electric machine having an arrangement as described above, the cost of manufacturing magnet segments 2, each having a curved surface with a radius of curvature of Rb, can be very high because of the elaboration required to produce such curved surfaces.
Although the use of a hollow and cylindrical ring magnet having a radial anisotropy in place of a segmented magnet has been proposed to bypass the problem of high manufacturing cost, such a ring magnet cannot induce a voltage having a sinusoidal waveform because of the existence of residual high harmonics. Consequently, the level of cogging will be considerably high.
It is, therefore, an object of the present invention to provide a rotating electric machine comprising a magnet having a radial anisotropy that can induce a voltage having a sinusoidal waveform with a low level of cogging.